468 ALASKAN GLACIER STUDIES
Mr. W. G. Weigle of the U. S. Forest Service has stated1 that "the absence of timber throughout upper Glacier Bay near the Muir Glacier and in many other places in southeast Alaska is so evidently the result of recent glaciation that I have been prone to attribute the absence of timber on the Copper River to the same cause, yet there may be other causes. One possible cause of this absence of timber worthy of consideration is that the spruce found near the mouth of the Copper Biver is tideland spruce (Picea sitckensis) and is only found near tidewater where there is a heavy rainfall. The spruce found on the Bremner, Tasnuna, Chitina and other portions of the upper Copper River basin is known as white spruce (Picea canadensis) which is one of the spruces found throughout the interior and other portions of Alaska but is not found in the excessively wet belt along the coast from Seward south. Another condition which may largely be responsible for the absence of timber in this region is that the Childs, Miles and Allen Glaciers even yet form a great barrier between the timbered regions of the upper and lower Copper River and the indications are that in quite recent times this ice field was much larger than it is at present, hence formed a greater barrier."
It seems probable that this distribution is due to an actual readvance of the ice tongues hi the forty mile strip, rather than a lingering of glaciers there. This is shown by the thick, mature forest north of Tasnuna River and in the Copper River basin near Chitina. Here all forest was certainly removed at the stage of maximum glaciation, for the ice was thick enough to extend above the timberline. We think it unlikely that the conifers readvanced to the Copper River basin by way of the deglaciated Copper River canyon because of the difference of species described by Weigle. It seems more probable that the unglaciated territory to the westward near Cook Inlet supplied this northern region with conifers. The lingering of conifers at the mouth of the Copper River canyon during the stage of TnaTHTnuin glaciation was made possible by the leaving of the driftiess area near Alagam'k and of driftiess areas to the west on Hinchinbrook Island, and probably also to the east in the Controller Bay coal field, where G. C. Martin2 states that at the maximum stage of Bering Glacier the ice rose only 200 to 700 feet above the present ice surface, which we interpret as below the timberline of that period.
The forty mile strip from which the conifers are absent is near the largest glaciers, and the present vegetation of the Copper River canyon is in an intermediate stage between the Prince William Sound region, where mature forest extends up close to the present glaciers, and the Yakutat Bay region, where the readvancing forest near the ice fronts is even less mature than in the Copper River canyon. Apparently the Heney, Allen, Miles, Childs, Goodwin, and other glaciers of this part of the Chugach Mountains advanced and destroyed the vegetation in part of the Copper River canyon at some, fairly recent period. We should not know of this if it was not for the absence of conifers in the forty mile strip between Tasnuna River and Childs Glacier.
Glaciation of the Lower Copper River Region. The present stage of glaciation of the lower part of the Copper River valley is one where the larger side valleys are filled with ice tongues, projecting into the main valley and expanding there as bulbs which obstruct the major drainage and supply Copper River with great amounts of water and sediment. A former stage, perhaps repeated, involved the occupation of this main valley by a
1 Personal communication, November 21, 1912. .
»Martin, G. C., Bull. 335, U. S. Geol. Survey, 1908, p. 50.